This invention relates to telecommunications networks, and more particularly the routing of signals and synchronization of terminals within a telecommunications network.
Most radio networks require some form of infrastructure. In the case of cellular and PCS phones, the infrastructure is in the base stations and the underlying network. For cordless phones, the twisted-pair telephone system provides the infrastructure. Other radio networks, such as the push-to-talk citizen""s band system, don""t require infrastructure but only supply a rudimentary service. In some instances where sophisticated voice and data networks are required the cost of installing infrastructure is prohibitive, only a temporary network is required, or there is not enough time to set up infrastructure. Some examples are remote communities, remote industrial job sites, emergency sites, sporting events, and conventions.
The system described by Lee et al. in U.S. Pat. No. 5,887,022 describes a system where a set of radio terminals activated in a region will self-configure into a communications network. This effectively produces the infrastructure within the radio units themselves. A drawback of this system is that all of the terminals must be in direct radio contact with every other terminal in the network. Ideally, this should not be a restriction, and terminals should be able to route their signal through a network of other terminals to reach destinations beyond their own radio range. Many routing algorithms exist, but most are based on wire/fibre network topologies where the connections between nodes are fixed. Some, such as the system described by Falmmer in U.S. Pat. No. 5,448,608, requires configuration information to be manually entered into each terminal, in this case geographic co-ordinates. No routing algorithm exists for a truly ad-hoc wireless network.
This invention is a radio terminal that, when activated within reasonable proximity of similar radios, becomes part of a self-organizing communications network. There are two classes of terminals in the network: active terminals and passive terminals. The active terminals form the backbone of the network by synchronizing to each other and establishing a common time domain multiple access (TDMA) frame in which all communication occurs. The active terminals also act as a network routers, forwarding signals between terminals that are too far apart to communicate directly. Passive terminals are simpler devices that can access the network but don""t participate in routing or synchronization. Battery powered devices are usually passive to conserve energy.
The network is designed so that no one terminal is critical for any network function, and therefore a failure in any terminal will not disrupt the remainder of the network. The network does not impose any specific modulation format on the signals passing through it. Any signal, analogue or digital that meets the network""s frequency, timing, and power specifications can be sent. On top of the networking base established by the terminals, a system can be built with whatever modulation formats, access protocols, congestion controls, addressing modes, etc. that the specific application requires.
Providing local communications to a small village is one example of how this invention can be applied. Each house in the village would have at least one active terminal that supports its part of the network backbone, as well as providing interfaces and connectors for wired devices within the home such as fixed telephones and computers. A household may also have one or more passive terminals like cordless phones and portable computers, which are able to roam throughout the community using any part of the network. The network time would be divided between one critical based protocol that handles the village""s local telephone system needs and a separate packet based protocol that handles the village""s data needs.
The active terminals divide the TDMA frame into a series of time slots. Some of the slots are designated data slots for carrying information between terminals, and others are designated synchronization slots. The data slots are further divided into sub-slots to accommodate routing. The source terminals transmits its signal during the first sub-slot and the remaining active terminals in the network use the following sub-slots to route the signal through the network.
The method of routing signals from source to destination used by this invention is different than the method of routing signals through a conventional cable or fiber network. A conventional network is composed of routing nodes with fixed links between the nodes. The nodes contain routing tables that direct the traffic to the appropriate links and the signal takes one path from the source to the destination. In this invention, the signal is allowed to take many paths through the network simultaneously. This method does not require routing tables and simplifies the routing algorithms considerably.
The routing algorithm works as follows. In the first sub-slot, the source terminal transmits the signal to the neighbouring terminals within its range. These terminals simultaneously rebroadcast it in the second sub-slot. The repeated signals reach both back towards the source terminal and further into the network, beyond the source terminal""s original range. The part that goes back towards the source is ignored, and the part that propagates further into the network is picked up by new terminals. These in turn rebroadcast it in the third sub-slot. With each successive rebroadcast, the signal is pushed further out into the network until it reaches every terminal, including the destination.
Such a routing scheme requires that each terminal have its TDMA slot and sub-slot boundaries synchronized to every other terminal in the network. Normally a single master clock sets the pace of the TDMA frame and rest of the radio terminals contain slave clocks that lock to this master. Since this particular network must operate without a base station or network controller, it does not have a single master clock. Instead all of the active terminals behave as both slave clocks and master clocks simultaneously to achieve network synchronization. All active teminals simultaneously transmit identical signals during special synchronization slots set aside in the TDMA frame. At random intervals, each terminal disables its transmitter and listens to the synchronization signals from the other terminals in order to measure and correct the time offset of its clock with respect to the rest of the network.
The function of the master clock is distributed throughout all of the active terminals. Each terminal measures the frequency offset between its internal reference clock and the rest of the network and then tries to move the frequency of the network toward its reference. The terminals do not try to influence the speed of the network directly by adjusting the frequency of their internal clocks; instead they influence the speed of the network indirectly by adjusting the transmission time of their synchronization signals. If a terminal transmits its synchronization signal earlier than usual, then the rest of the network will react by speeding up. If it transmits later, the rest of the network will react by slowing down. Effectively, each terminal applies a synchronization xe2x80x9cforcexe2x80x9d to the rest of the network. When the forces pushing to go slower balance the forces pushing to go faster, the network assumes a stable frequency.
These and other aspects of the invention are found in the detailed description and the claims that follow.